Circuit for controlling exposure

ABSTRACT

A circuit for controlling exposure which has a Miller integrator for integrating a photocurrent from a photoelectric converting light acceptor, a circuit for generating a predetermined set level, and a level comparator for generating an exposure finishing signal when the output of the integrator arrives at the predetermined level set by the level generating circuit. Thus, a great deal of electric current can be obtained without using a photoelectric tube or thyratron. A low impedance circuit may be used which is not affected by leaking current or dark current so as not to deteriorate the elements and thereby improve circuit reliability.

United States Patent 11 1 1111 3,790,789 Takahashi et al. Feb. 5, 1974[54] CIRCUIT FOR CONTROLLING EXPOSURE Primary Examiner.lames W. Lawrence[76] inventors: lsao Takahashi; Tsutomu Klmura, Assist, Examiner-T,Grigsby both of 210 Minami Ashigarwshi Attorney, Agent, or Fzrm-Sughrue,Rothwell, Mion,

Kana Ja Zinn and Macpeak [22] Filed: Sept. 22, 1972 21 Appl. N0.:291,481 [57] ABSTRACT 1 A circuit for controlling exposure which has atMiller [30] Foreign Application priority Data integrator for integratinga photocurrent from a photoelectric converting light acceptor, a circuitfor generating a predetermined set level, and a level comparator forgenerating an exposure finishing signal when the output of theintegrator arrives at the predetermined level set by the levelgenerating circuit. Thus, a great deal of electric current can beobtained without [561 Refml'ces CM 3311i 1523?? ilfi fe iie fivillfiiffi $1 3535? UNITED STATES PATENTS leaking current or dark current soas not to deterio- 3,504,603 4/1970 Brzonkala et al. 250/214 P rate theelements and thereby improve circuit teliability.

Sept. 22, 1971 Japan ..46/74031 [52] US. Cl. 250/209, 250/214 P, 355/68[51] Int. Cl. G03b 2 7/74, G03b 27/78 [58] Field of Search 250/214 P,209; 355/68 2 Claims, 5 Drawing Figures LEVEL B COMPARATOR OUT OPER. AMPl0 PATENTEU FEB 5 I974 LEVEL COMPARATOR MILLER INTEGRATOR LIGHT ACCEPTORLEVEL GEN. CIRCUIT COMPARATOR 75in LEVEL PER. AMP

FIG 3 FIG. 4

DENSITY FIG. 5 v

BACKGROUND OF THE INVENTION 1. Field of the Invention This inventionrelates to an automatic photographic printer, and more particularly to acircuit for controlling the exposure for an automatic photographicprinter having a control function for imparting optimum exposure for thecharacteristic of the photosensitive material usedfor printing.

2. Description of the Prior Art The density of a negative of an originalpicture for photographic printing is different for every negative, andsince it is difficult to hold the illumination of a source of lightconstant during exposure to give a constant amount of light to thephotographic sensitive material, the exposure is controlled by a lightamount integrating circuit for integrating with respect to time thelight to be given to the photographic sensitive material.

In prior art circuitry an integrating circuit is connected in serieswith a photoelectric tube or photomultiplier to produce a chargingcurrent for the integrating circuit. A triggering device such as athyratron is also connected to the integrating circuit such that itfires when the output of the integrating circuit reaches a predeterminedlevel. In circuits of this type the triggering device must have a highinput impedance to allow the charging of the integrating circuit.However, due to the necessity of the high impedances, the circuit willbe affected by the humidity of the environment. When the humidity ishigh the charging current is cancelled by a leakage current, thuscausing the improper operation of the device.

SUMMARY OF THE INVENTION The present invention contemplates theelimination of the aforementioned disadvantages of the conventionalexposing circuit of a photographic printer by providing a novel andimproved circuit for controlling exposure.

It is, therefore, an object of the present invention to provide acircuit for controlling exposure for an automatic photographic printerwhich may simply adjust the intensity of printing.

It is another object of the present invention to provide a circuit forcontrolling exposure for an automatic photographic printer which mayprovide variable compensation depending upon the sensitivity of theprinting paper or intensity of printing.

It is a further object of the present invention to provide a circuit forcontrolling exposure for an automatic photographic printer which is notaffected by leaking current or dark current, thus improving thereliability.

One characteristic feature of the present invention resides in a circuitfor controlling exposure which has a Miller integrator for integrating aphotocurrent from a photoelectric converting light acceptor, a circuitfor generating a predetermined set level, and a level comparator forgenerating an exposure finishing signal when the output of theintegrator arrives at the predetermined level set by the levelgenerating circuit.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a circuit diagram of theconventional exposure control circuit.

FIG. 2 is a block diagram explaining the circuit for controllingexposure in accordance with the present invention.

FIG. 3 is a basic circuit diagram of a Miller integrator.

FIG. 4 is a circuit diagram for explaining generally the presentinvention.

FIG. 5 is a graph showing the characteristic of the concentration of thenegative and printing exposing time.

DESCRIPTION OF THE PREFERRED EMBODIMENT The basic example of theconventional circuit for controlling the exposure using the light amountintegrating circuit will now be described simply with refer- ?MPJQEIG- Aswitch 1 is opened with the starting of the exposure. The lightpenetrating the negative contacts a photoelectric tube orphotomultiplier 2 to produce a photocurrent, which charges timingcondenser 3. The bias voltage of the integrating condenser 3 is switchedfrom negative to positive by the charging photocurrent so that when thegrid voltage V, of a thyratron 4 reaches zero volts, the thyratron istriggered and an exposure finishing relay 5 is operated to terminate theexposure Since the integrating condenser 3 of this type of circuit isdirectly connected to the photoelectric converting element such asphotoelectric tube 2, in order to obtain an output from the integratingcircuit, it must be connected to a high impedance element such as thegrid of the thyratron 4, or its impedance must be converted. If theconnection has a high impedance it is affected by the humidity of theenvironment, and when the humidity is high such as in a dark room, thecharging photocurrent is cancelled by the leaked current of the circuitelements such as switch 1. Thus, even if proper time has lapsed, theexposure does not finish. Further, such a conventional circuit mustprovide a separate slope circuit in order to compensate for thereciprocity law of the photosensitive material.

The present invention has a low impedance circuit and by utilizing alarge photocurrent to the control of the exposure, so as to vary theintegrating time constant of the photocurrent integrator in response tothe density of the negative to be printed to thus positively and simplyprovide a slope circuit for improving the exposing time characteristicswith respect to the density of the negative.

The Miller integrator, which is known per se as a component element, isshown in FIG. 3 in the circuit of the present invention. The circuitstarts to integrate when the start switch 16 is opened. The timeconstant of the integrating potential of the time 0 to T is determinedby a condenser C and a resistor R. The relationship between the inputvoltage Vi and the output voltage vc of the mirror integrator 1 T Vc=-,JI Vtdt FIG. 4 shows one example of the embodiment of the presentinvention. The circuit comprises an operational amplifier 10,photoconductive elements 11 and 13, variable resistors 12 and 14,condenser and start switch 16. The output of the operational amplifieris applied to the level comparator 8.

The operation of the circuit of the present invention will now bedescribed with reference to the example shown in FIG. 4. The followingdiscussion is based on the assumption that the photoconductive element13 has a certin value for one negative. Of course, this value will varywhen a different negative is inserted.

Since the condenser 15 is short-circuited by the start switch 16 beforethe starting of exposure, the potential E, at the point B is held at 0.If the switch 16 is openedv at the same time of the starting of theexposure, the potential E,, at the point A is integrated so that thepotential E at the point B is gradually increased. The photoconductiveelement 11 detects the density of the negative and the resistance R11 isdetermined by each negative and remains constant for a particularnegative unless the source of light for printing is varied duringexposure. Therefore, the potential E,, at the point A is constant duringexposure for each negative. If the resistance value R13 of thephotoconductive element 13 is made constant regardless of the density ofthe negative in such state, then, if a fixed resistor is put instead ofthe photoconductive element 13, the potential IE at the point B after 1hours is expressed as follows from the formula (I):

s A/K)r In operation, both resistance R11 and R13 vary in accordancewith the density of the negative. A variation in resistance R11 changesthe potential at point A, whereas a variation in resistance R13 changesthe time constant of the Miller integration.

Here, when the set predetermined level applied to the level comparator 8is represented by E2, and when the potential E of the formula (2)reaches E2, it generates the exposure finishing signal. Therefore, theexposing time T is obtained by the following formula:

In the above formula, K is a factor which does not vary according to thedensity of the negative being processed.

As to the relation of the density D of the negative to the exposing timeT, when an overexposed negative is printed, since the light penetratingthe negative is reduced, the incident light on the photoconductiveelement 11 becomes low, and accordingly the resistance value R11 becomeslarge. Therefore, the potential 13,, at the point A is lowered, whilethe exposing time T is extended. In case of under exposure, for thenegative, the above operation is entirely opposite, and the potential E,at the point A is increased, but the exposing time T becomes short. Thisstate is represented by the line (a) in the graph shown in FIG. 5. Thegradient of this characteristic curve (a) depends upon thecharacteristic of the intensity of illumination versus resistance of thephotoconductive element 11. Since the constant of the integration isconstant only if a preamplifier is provided between the light acceptor(photoelectric converting element such as photoconductive element, etc.)and the Miller integrator, varying gain only moves the characteristic ofthe density to the exposing time up and down in parallel, but the slopedoes not change.

In the above operation as described, the resistance value of thephotoconductive element 13 was assumed to be constant. If, however, thephotoconductive element 13 is placed at the position to change inresponse to the density of the negative similar to the element 11, ifthe resistance value of the variable resistor 14 is represented by R14,the capacitance of the condenser 15 is represnted by C, the formula (2)is expressed as follows:

Therefore, the formula (3) is expressed as follows:

= -(E /E,,) (R13 R14)C the value of the resistance R13 in comparisonwith the resistance R14 of the variable resistor 14, the greater theeffect on the extension of the exposure finishing time, as is shown bythe characteristic curve (b) or (c) in the graph shown in FIG. 5. If theresistance R14 is sufficiently larger than the resistance R13, thevariation of the resistance R13 of the photoconductive element 13 can beignored, and accordingly it approaches the characteristic curve (a) inthe graph shown in FIG. 5. Therefore, the slope can be simply changed byproviding the value of the variable resistor 14 so as to adjust theintensity of printing.

Further, if the respective values of the applied voltage E1 of thephotoconductive element 11, the capacity of the condenser 15 of theMiller integrator, and the set predetermined level E2 of the levelcomparator are varied, the compensation keys for the compensation basedon the sensitivity of the printing paper to be printed or thecompensation for the degree of the intensity of printing are providedindependently so as to variably compensate the printing exposure.

The compensation keys represent means for varying the exposureconditions according to the sensitivity of the printing paper, etc.(manual adjustment), and do not vary conditions in response to change inthe density of the negative of the original picture. Depending upon thefactor compensated for, the compensation keys could correspond to avariable resistor for varying the level E2 (not shown), a variableresistor such as R11 or a variable condenser such as C.

Since the circuit of the present invention has the feature that it doesnot use a photoelectric tube or thyratron, a low impedance circuit maybe used and a great deal of current may be obtained. Accordingly, thecircuit is not affected by the leaked current or dark current, anddeterioration of the elements is not present, thus improvingreliability.

While the invention has been particularly shown and described withreference to the preferred embodiment thereof, it will be understood bythose skilled in the art that various changes in form and details may bemade therein without departing from the spirit and scope of theinvention.

We claim:

1. A circuit for controlling the exposure of a photographic printthrough a negative comprising:

a. first and second photoelectric converting light acceptor means forgenerating first and second photocurrents, respectively, in accordancewith the light passing through said negative;

b. first and second resistor means respectively connected in series withsaid first and second photoelectric converting light acceptor means, theseries connection of said second resistor means and said secondphotoelectric converting light acceptor means being connected to thejunction of said first resistor means and said first photoelectricconverting light acceptor means;

c. Miller integrator means including an operational amplifier, capacitormeans connected between the input and output of said operationalamplifier, and switch means in parallel with said capacitor meanswhereby upon the opening of said switch means said capacitor is charged,said series connection of said second resistor means and said secondphotoelectric converting light acceptor means being connected as theinput resistance of said operational amplifier thereby determining theintegrating time constant of said Miller integrator;

(1. level generator means for generating a signal having a predeterminedlevel; and

e. level comparator means for comparing the output of said Millerintegrator means to said predetermined level and for generating anexposure finishing signal when the output of said Miller integratorreaches said predetermined level.

2. The circuit as set forth in claim 1 wherein said first and secondresistor means are variable resistances.

v UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION February 5, 1974Patent No. 9 89 I Dated Inventor (s) Isao TAKAHASHI et a1 It iscertified that error appears in the above-identified patent and thatsaid Letters Patent are hereby corrected as shown below:

In the Heading:

Insert Assignee's name and address as follows:

Fuji Photo Film Co. Ltd. Ashigara-shi,

Kanagawa, Japan (SEAL) Attest:

MCCOY M. GIBSON; JR. Attesting Officer C. MARSHALL DANN Commissioner ofPatents fORM PC4050 (10459) USCOMM-DC wave-pea U.$. GOVERNMENT PRINTINGOFFICE: I," 0-355-33,

' UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION February 5, 1974Patent: No. 9 9 Dated Inventorks) Isao TAKAHASHI et a1 It is certifiedthat error appears in the above-identified patent and that said LettersPatent are hereby corrected as shown below:

In the Heading:

Insert Assignee's name and address as follows:

- Fuji Photo Film Co. Ltd. Ashigara-shi,

Kanagawa', Japan Signed and sealed this 15th day of August 197 (SEAL)Attest:

MoCOY M. GIBSON; JR. C. MARSHALL DANN Attesting Officer Commissioner ofPatents FORM PO-1050 (1-0-69) UsCOMM-DC wave-pee t (1.5. GOVERNMENTPRINTING OFFICE: I959 0-366-335.

1. A circuit for controlling the exposure of a photographic printthrough a negative comprising: a. first and second photoelectricconverting light acceptor means for generating first and secondphotocurrents, respectively, in accordance with the light passingthrough said negative; b. first and second resistor means respectivelyconnected in series with said first and second photoelectric convertinglight acceptor means, the series connection of said second resistormeans and said second photoelectric converting light acceptor meansbeing connected to the junction of said first resistor means and saidfirst photoelectric converting light acceptor means; c. Millerintegrator means including an operational amplifier, capacitor meansconnected between the input and output of said operational amplifier,and switch means in parallel with said capacitor means whereby upon theopening of said switch means said capacitor is charged, said seriesconnection of said second resistor means and said second photoelectricconverting light acceptor means being connected as the input resistanceof said operational amplifier thereby determining the integrating timeconstant of said Miller integrator; d. level generator means forgenerating a signal having a predetermined level; and e. levelcomparator means for comparing the output of said Miller integratormeans to said predetermined level and for generating an exposurefinishing signal when the output of said Miller integrator reaches saidpredetErmined level.
 2. The circuit as set forth in claim 1 wherein saidfirst and second resistor means are variable resistances.